The district includes Chino, south Montclair, south Ontario and areas of unincorporated land in Chino Hills and the Dairy Preserve.

The County Department of Public Health recommends residents eliminate all standing water on their property, including birdbaths, green swimming pools, ponds, old tires, buckets, flower pots, clogged gutters, or puddles from leaky sprinklers.

Mosquitoes are most active at dusk and dawn, so the department recommends not spending much time outside during those hours.

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Health officials also recommend applying the insect repellent DEET according to manufacturers directions.

Symptoms of West Nile virus usually occur two to 15 days after infections, according to health officials. The Center for Disease Control and Prevention says that 70 percent to 80 percent of those infected develop no symptoms at all, and are even unaware that they’re infected.

About 1 in 5 people who are infected will develop a fever with other symptoms — headache, body aches, joint pain, vomiting, diarrhea, or rash, according to the CDC. Most people with this recover completely, but fatigue and weakness can last for weeks or months.

In the most serious West Nile virus cases, a severe headache, stupor, disorientation, tremors, convulsions and muscle weakness can occur as well as paralysis and coma, according to a statement from the West Valley Mosquito and Vector Control District. The CDC says that only about 1 percent of those infected with the virus develop these serious symptoms.